In oil and gas production, wells are drilled using a rotary driling method with the cutting bit attached to a drill stem while drilling mud is circulated to the bottom of the bore hole and back to the surface to remove the cuttings made by the bit. These drilling fluids can be of various types, but the most common fluid has an aqueous base in which colloidal solids are suspended. These solids are special forms of clay, most commonly Wyoming Bentonite or similar types of clay which produce the desired viscosity and thixotropic properties for efficient removal of cuttings from the bore hole.
Some of the most serious problems encountered in producing and maintaining effective clay-based aqueous drilling muds are caused by the interaction of the mud with the earth formation being drilled. These include the possible contamination of the mud by fluids in the earth formation and the incorporation into the mud of viscosity producing inert solids. Also, the drilling mud is heated because of high temperatures in the formation, particularly in wells exceeding 5,000 feet in depth. In addition to such problems, control of the properties of the drilling mud is complicated by loss of water from the drilling fluid to the formation being drilled. If too much water is lost, the downhole formation can be destablized with resultant collapse of the hole.
These problems are even more acute when drilling a formation which has high salinity, or if the mud is contaminated by divalent ions such as calcium and magnesium. In such situations, the clay, especially bentonite, tends to thicken the mud uncontrollably and the drilling fluids tend to lose considerable water to the formation. Also, there is a need to formulate drilling fluidswith sea water for off-shore drilling. To address these problems, various types of polymers have been incorporated in the drilling muds in order to control the viscosity, to stabilize the mud under high temperature conditions and in the presence of various minerals, and to reduce the tendency of the mud to lose water into the formation.
In summary, the most important functions of a drilling fluid are to (1) remove formation cuttings from the bottom of the hole and transport them to the surface, (2) provide sufficient hydrostatic pressure against the formation fluids, (3) stabilize downhole formations and prevent hole collapse, (4) prevent loss of fluid to permeable formations, (5) cool and lubricate the bit and drill string, and (6) help suspend the weight of the drill string casing.
U.S. Pat. No. 3,764,530 (1973) describes a drilling fluid which contains an acrylic acid copolymer which does not contain halogen, but has an average molecular weight of below about 2500. Drilling muds are described which can be prepared from fresh water, brine or salt water and contain suspended solids such as clay bentonite and a chemical dispersant, termed a "thinner". The thinner deflocculates the clay and drilled solids and the acrylic acid polymer is added to reduce thermal degradation.
U.S. Pat. No. 3,730,900 (1973) describes an additive for drilling fluids which is a low molecular weight copolymer of styrene sulfonic acid and maleic anhydride, which is said to stabilize the viscosity characteristics and improve the clay suspension.
U.S. Pat. No. 4,476,029 (1984) discloses the use of polyacrylic acid as a dispersant in water-based bentonite drilling muds which also contain a weighting agent plus a commercial waterloss control additive.
U.S. Pat. No. 4,680,128 (1987) discloses the use of low molecular weight copolymers of acrylic acids and salts of vinylsulfonic acid as a dispersant and deflocculant for stabilizing aqueous clay-based drilling fluids which are subjected to calcium contamination. This additive is also said to help prevent thermal degradation.
U.S. Pat. No. 4,699.722 (1987) discloses the use of polymers such as dimethylaminopropyl methacrylamide as aviscosifer in completion fluids for oil wells, which can include non-damaging drilling fluids. This additive is said to increase viscosity of the fluid while remaining stable at high temperatures.
European Pat. Application, Publication No. 0 120 592 (1984) describes stabilizing fines in permeable subterranean formations with certain organic polycationic polymers containing 2 quarternary ammonium moieties in the polymer repeating unit.
The above described polymer additives may improve mud rheology but not fluid retention.
The problem of loss of water into the formation from drilling muds is addressed by U.S. Pat. No. 4,533,708 (1985) which suggests as a waterloss control additive, copolymers of a carboxylic functional monomer, an acrylamide-type monomer and a cationic-containing monmoer, e.g., acrylic acid, acrylamide and dimethyldiallyl ammonium chloride. U.S. Pat. No. 4,652,623 (1987) also deals with the prevention of fluid loss by using similar copolymers such as those of acrylic acid, 2-acrylamido-2-methyl propyl sulfonic acid, dimethydiallyl ammonium chloride and acrylamide.
The polyacrylamides and copolymers consisting primarily of polyacrylamide have been used as coagulents in various systems such as disclosed in U.S. Pat. No. 3,278,506 (1966). U.S. Pat. No. 3,957,739 (1976) describes the use of inverse emulsion polymerization to form polyacrylamides said to have very high molecular weight useful as flocculation agents. These polymers are shown to reduce the sedimentation time of kaolin.
Polyvinylamine hydrochloride, having a molecular weight of about 3.times.10.sup.5 on greater, is described in U.S. Pat. No. 4,217,214 (1980), as effective for use as a flocculating agent in wastewater systems. The addition of the polyvinylamine is said to improve filtration rates.
U.S. Pat. No. 4,444,667 (1984) discloses a homopolymer of N-vinylformamide which has been hydrolyzed so that from 10 to 90% of the formyl groups are converted to amine groups. It is said that this polymer is useful as a flocculant in sludges and increases the size of the particles of solids to be flocculated.
GB 2,152,929 (1985) describes a process for making N-substituted formamides which can be converted to N-vinylformamides, useful as a monomer to prepare polymers which can be hydrolyzed to polyvinylamine. This polymer can then be used for dehydrating organic sludges and improving the filterability or yield of fillers in papermaking.
U.S. Pat. No. 4,500,437 (1985) describes how friction can be reduced in fracture-acidizing an oil or gas well by adding to the acidizing fluid acrylamide copolymers or terpolymers containing N-vinylformamide and N-vinylacetamide. These copolymers can contain from 5 to 50% by weight of N-vinylformamide, 10 to 95% acrylamide and up to 85% of third monomers which can include N-vinylacetamide. The molecular weight of these copolymers can range from 20,000 to 15.times.10.sup.6. The statement is made that although hydrolytic degradation occurs on dependent groups of the polymer, the products remain acid soluble and will not precipitate from solution. There is, however, no teaching in regard to hydrolysis of the N-vinylamide groups to vinylamine units. Also, this patent does not address the problem of waterloss control in drilling muds, nor does it suggest the use of hydrolyzed co-polymers as waterloss preventors in clay-based drilling fluids.